Memory device program window adjustment

ABSTRACT

In one or more embodiments, a memory device is disclosed as having an adjustable programming window having a plurality of programmable levels. The programming window is moved to compensate for changes in reliable program and erase thresholds achievable as the memory device experiences factors such as erase/program cycles that change the program window. The initial programming window is determined prior to an initial erase/program cycle. The programming levels are then moved as the programming window changes, such that the plurality of programmable levels still remain within the program window and are tracked with the program window changes.

RELATED APPLICATION

This is a continuation of U.S. application Ser. No. 11/873,894, titled“MEMORY DEVICE PROGRAM WINDOW ADJUSTMENT,” filed Oct. 17, 2007 (allowed)and is commonly assigned and incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates generally to semiconductor memory andmore particularly to non-volatile memory devices.

BACKGROUND

Memory devices are typically provided as internal, semiconductor,integrated circuits in computers or other electronic devices. There aremany different types of memory including random-access memory (RAM),read only memory (ROM), dynamic random access memory (DRAM), synchronousdynamic random access memory (SDRAM), and non-volatile/flash memory.

Flash memory devices have developed into a popular source ofnon-volatile memory for a wide range of electronic applications. Flashmemory devices typically use a one-transistor memory cell that allowsfor high memory densities, high reliability, and low power consumption.Common uses for flash memory include personal computers, personaldigital assistants (PDAs), digital cameras, and cellular telephones.Program code and system data such as a basic input/output system (BIOS)are typically stored in flash memory devices for use in personalcomputer systems.

Each cell in a non-volatile memory device can be programmed as a singlebit per cell (i.e., single level cell—SLC) or multiple bits per cell(i.e., multilevel cell—MLC). Each cell's threshold voltage (V_(th))determines the data that is stored in the cell. For example, in an SLC,a V_(th) of 0.5V might indicate a programmed cell while a V_(th) of−0.5V might indicate an erased cell. The MLC has multiple positiveV_(th) distributions that each indicates a different state whereas anegative distribution typically indicates an erased state. MLC takeadvantage of the analog nature of a traditional flash cell by assigninga bit pattern to a specific voltage range stored on the cell. Thedistributions are part of a larger programming window (i.e., the voltagerange in which a memory device is programmable) and are separated by avoltage space or margin that is relatively small due to the limitationsof fitting, for example, four states into a low voltage memory device.

As flash memory cells go through multiple erase/program cycles, theylose their ability to be erased to a specific negative voltage. This isa result of electron traps in the tunnel oxide that separates the chargestorage layer (e.g., floating gate) from the substrate. This isespecially undesirable in MLC technology where more states are requiredto be stored within the programming window. A smaller programming windowafter cycling can limit the quantity of MLC program states available inthe memory device.

FIG. 1 illustrates a graph showing the programming threshold voltagechanges for a typical non-volatile memory device. This figure shows thethreshold voltages of the non-volatile memory cells along the y-axis andthe quantity of erase/program cycles along the x-axis.

Initially, a memory cell might have a maximum erased state of −3.0V anda maximum programmed state of 3.0V as shown closer to the y-axis wherethe quantity of erase/program cycles is low. As the cycles approach 10 kcycles, it can be seen that the maximum erase state threshold voltagehas increased to 0V while the maximum programmed state threshold voltagehas increased to 5.0V. Therefore, the most reliable program window inthis graph is fixed as being between 0V and 3V. This is the fixedprogram window that is used for sense operations for a non-volatilememory device.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art fora way to adjust for the effects of erase/program cycling on a programwindow in a non-volatile memory device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a graph of memory cell threshold voltages (V_(t)) versusquantity of erase/program cycles.

FIG. 2 shows a flowchart of one embodiment of a method for programwindow adjustment while programming.

FIG. 3 shows a flowchart of one embodiment of a method for dataretrieval at adjusted program levels.

FIG. 4 shows one embodiment of a level conversion table in accordancewith the methods of FIGS. 1 and 2.

FIG. 5 shows a flowchart of an alternate embodiment of a program windowadjustment method.

FIG. 6 shows a block diagram of one embodiment of a memory systemincorporating the method for program window adjustment of the presentdisclosure.

DETAILED DESCRIPTION

In the following detailed description of the invention, reference ismade to the accompanying drawings that form a part hereof and in whichis shown, by way of illustration, specific embodiments in which theinvention may be practiced. In the drawings, like numerals describesubstantially similar components throughout the several views. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. Other embodiments may be utilizedand structural, logical, and electrical changes may be made withoutdeparting from the scope of the present invention. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present invention is defined only by the appendedclaims and equivalents thereof.

FIG. 2 illustrates a flowchart of one embodiment of a method for programwindow adjustment while programming. A block erase operation isperformed 201 on the memory block to be programmed. The erase operationis comprised of making the word line coupled to each row of memory cellsmore negative than the substrate. For example, biasing each word linewith an erase pulse comprising a large negative voltage (e.g., −20V)could be used to move the threshold voltages of the memory cells in theblock to a negative level. After each erase pulse, an erase verifyoperation is performed to determine if the memory cells are adequatelyerased.

The erase threshold voltages of the erased memory block are then read203 with a sensing operation. This provides the memory controller withthe new erase threshold voltages for the memory block. In oneembodiment, the maximum erase threshold voltage for each memory cell isidentified as a reliable erase threshold voltage for the memory block205. This threshold is used to generate a level conversion table 207 asillustrated in FIG. 4 and discussed subsequently.

The maximum reliable program threshold is identified as the minimumprogramming threshold for the memory device. In other words, the maximumprogramming threshold at a given point in the life of a memory device,say 4V, might not be a reliable threshold voltage for programming sincethat high of a voltage might not be programmable to an accurate enoughlevel over the life of the device. Thus, a reliable threshold voltagethat can be accurately programmed over the life of the device isidentified as the maximum programming threshold.

The updated level conversion table is used to program referencethreshold voltage levels into reference cells that are associated withthe erased memory block and/or level conversion data is programmed intoa memory location 209. If the memory device operates in the digitaldomain, each reference cell is programmed in accordance with thereference fixed bit pattern that is associated with the updatedconverted data bit pattern representative of each programmable level, asillustrated in the fourth column of FIG. 4.

In one embodiment, only one reference cell is used per programminglevel. Alternate embodiments can use other quantities of referencecells. For example, one embodiment might use multiple reference cellsper programming level and take the average of the voltages programmedinto the cells for sensing purposes, after removing the highest andlowest threshold reference cell from the averaging process.

In another embodiment, conversion information of the updated levelconversion table for the erase block is programmed in memory cells. Thememory cells with the conversion information can be in the same block oranother predetermined location. The conversion information is read fromthis location with every read of the memory block and the conversiontable is used to retrieve actual data bits from the read data bits.

One embodiment of a level conversion table is illustrated in FIG. 4. Thefirst column 401 is comprised of the threshold voltage levels thatconstitute the range of threshold voltages for the particular memorydevice. This table starts at a maximum erased level of −3.75V and goesup to a maximum programmed level of 4.00V in increments of 0.25V.

The second column 402 lists a reference state, each being assigned adistinct 5-bit fixed bit pattern, for each of the different thresholdvoltage levels for the memory device. The most negative thresholdvoltage is assigned a bit pattern of “00000” and the most positivethreshold voltage in the threshold voltage range is “11111”. Each bitchange represents the 0.25V threshold voltage increment.

The reference fixed bit patterns are generated by the memory arraycontroller and are used in an embodiment that operates in the digitaldomain. In other words, when a read operation is performed on the memoryarray, the array outputs a signal corresponding to one of the five-bitreference fixed bit patterns for each cell that is read, instead of athreshold voltage. In an alternate embodiment that operates in theanalog domain, the actual threshold voltage of each memory cell beingread is output instead of the digital bit pattern.

The third column 403 of the table of FIG. 4 lists the converted data bitpatterns that are accurate prior to any erase/program cycling. Thesefour-bit, pre-cycle bit patterns are each representative of a particularreference digital fixed bit pattern of the memory device and eachcorresponds to a distinct one of the five-bit “reference” patterns. Forexample, the pre-cycle converted data bit pattern corresponding to“00101” is “0000”. This is also the pre-cycle converted bit pattern thatis representative of the erased level of the memory cell having athreshold voltage level of −2.50V. The pre-cycle converted data bitpattern corresponding to “11000” is “1111” and represents a thresholdvoltage of 2.25V.

In the embodiment of FIG. 4, the programmed state represented by“1111”corresponds to the maximum programmed threshold voltage level for thememory cells of the memory block and represents the top of the programwindow. The bit pattern “0000” corresponds to the erased level of thememory block and represents the bottom of the program window.

The fourth column 404 of the table of FIG. 4 lists the converted databit patterns that might occur in a post-cycling scenario. It can be seenthat the bottom of the program window (i.e., “0000”) has shifted up tocorrespond to the fixed bit pattern “01001” that is representative ofthe identified reliable threshold voltage level (e.g., −1.50V). The topof the program window (i.e., “1111”) has shifted up to correspond to thefixed bit pattern “11100” that is representative of a threshold voltagelevel of 3.25V.

The program level conversion table of FIG. 4 could be generated by thememory controller and stored in memory for future use during memory readoperations. The table, in one embodiment, is updated at every eraseoperation. Alternate embodiments can update the table at differentintervals. The constant updates allow the memory controller to track thechanges in the memory cells as they are cycled and to adjust the programwindow accordingly.

FIG. 3 illustrates a flowchart of one embodiment of a method for dataretrieval. This method uses the adjusted program levels from the columnof converted data bit pattern 404 of FIG. 4.

The memory block reference cells that were programmed with the adjustedprogram levels are read 301. These reference cell bit patterns are usedto generate a conversion table 303 that is used during the reading ofthe data cells. The conversion table contains the differences betweenthe read reference cell bit pattern and the expected reference cell bitpattern. The expected reference cell bit pattern is the bit pattern thatwas initially assigned to each threshold voltage level of the memorycell prior to any erase/program cycles.

For example, an initial converted data bit pattern (Pre-Cycling) of“0000”, according to FIG. 4, is assigned to −2.50V and Fixed Bit Patternof “00101” (i.e., expected reference cell bit pattern). After cycling,the “0000” state is now assigned to −1.50V and a Fixed Bit Pattern of“01001” (i.e., read reference cell bit pattern). This provides a 1.0Vreference cell bit pattern difference that is stored in the conversiontable.

When the data cells are read 305, the data is adjusted for differencesin the program window as indicated by the reference cell bit patterndifferences that are stored in the conversion table 307. If the memoryarray of the memory device operates in the digital domain, thecontroller reads a digital signal corresponding to a four bit digitalbit pattern for each memory cell read to determine the programmed statesof the cells.

The method illustrated in FIG. 3 provides a sensing operation with theability to adjust data read from memory cells with changes in theprogramming window as the memory device experiences increasingquantities of erase/program cycles. As shown in the level conversiontable of FIG. 4, data read from a memory cell prior to any cycling isgoing to have a different reference fixed bit pattern associated with agiven threshold level than data read after a number of cyclingoperations.

FIG. 5 illustrates a flowchart of an alternate embodiment of the programwindow adjustment method previously discussed. The embodiment of FIG. 2adjusts the program window by tracking the erase levels. The alternateembodiment of FIG. 5 adjusts the program window by tracking both eraseand programmed levels.

Since this method also tracks programmed levels, it begins by performinga block erase 500 followed by a program operation 501. The programoperation is performed until the maximum programming level of the deviceis achieved. This maximum threshold voltage is then read out 503. Ablock erase operation is then performed 505 to achieve a maximum erasethreshold voltage. The erase threshold voltages are then read out 507.

The maximum reliably programmable threshold voltage and the maximumreliably erasable threshold voltage are then used to generate a usableprogram window for the memory device 509. As in the previous embodiment,this window then defines the programmable range of the memory array suchthat all of the programmable levels should fit inside the window.

The margins between the programmable levels, as represented by bitpatterns, are adjusted so as to maximize the use of the availableprogram window 511. In other words, once the upper and lower boundariesare established by the minimum program threshold and the maximum erasethreshold, the programmable states (as represented by bit patterns) aredistributed throughout the window. The controller can also establish thenumber of program levels that would reliably fit in the window prior toactual programming of the block of memory cells. If a conversion tableis used, it can be generated at this point as well and stored in memory513. In an alternate embodiment, reference cells that are associatedwith the erased block are programmed with the adjusted program levels513. In still another embodiment, both the conversion table is generatedand the reference cells are programmed 513.

Data read out of the memory cells are adjusted based on the adjustedprogram window. This is accomplished by generating a conversion tablefrom the expected reference cell bit pattern and the actual bit patternread from the reference cells. The conversion table can also begenerated from reading the conversion information stored in memory in analternate embodiment. The difference between the initial reference fixedbit pattern and the recently adjusted reference cell bit patterns isthen determined. The conversion table is applied to the read data bitpatterns from the memory cells to get the actual data.

The embodiment illustrated in FIG. 5 can be performed by the memorycontroller of the memory device. In one embodiment, it is performedprior to every program operation. Alternate embodiments perform themethod after a certain number of erase/program cycles have beenperformed.

FIG. 6 illustrates a functional block diagram of a memory device 600that can incorporate the memory cells of the present invention. Thememory device 600 is coupled to a processor 610. The processor 610 maybe memory controller, a microprocessor or some other type of controllingcircuitry. The memory device 600 and the processor 610 form part of amemory system 620. The memory device 600 has been simplified to focus onfeatures of the memory that are helpful in understanding the presentinvention.

The memory device includes an array of non-volatile memory cells 630that can be flash memory cells or other types of non-volatilesemiconductor cells. The memory array 630 is arranged in banks of rowsand columns. The control gates of each row of memory cells is coupledwith a wordline while the drain and source connections of the memorycells are coupled to bitlines. As is well known in the art, theconnection of the cells to the bitlines depends on whether the array isa NAND architecture or a NOR architecture. The memory cells of thepresent invention can be arranged in either a NAND or NOR architecture,as described previously, as well as other architectures.

An address buffer circuit 640 is provided to latch address signalsprovided on address input connections A0-Ax 642. Address signals arereceived and decoded by a row decoder 644 and a column decoder 646 toaccess the memory array 630. It will be appreciated by those skilled inthe art, with the benefit of the present description, that the number ofaddress input connections depends on the density and architecture of thememory array 630. That is, the number of addresses increases with bothincreased memory cell counts and increased bank and block counts.

The memory device 600 reads data in the memory array 630 by sensingvoltage or current changes in the memory array columns using senseamplifier/buffer circuitry 650. The sense amplifier/buffer circuitry, inone embodiment, is coupled to read and latch a row of data from thememory array 630. Data input and output buffer circuitry 660 is includedfor bi-directional data communication over a plurality of dataconnections 662 with the controller 610. Write circuitry 655 is providedto write data to the memory array.

Control circuitry 670 decodes signals provided on control connections672 from the processor 610. These signals are used to control theoperations on the memory array 630, including data read, data write, anderase operations. The control circuitry 670 may be a state machine, asequencer, or some other type of controller. The control circuitry 670is adapted to perform the programming window adjustment embodimentsdisclosed previously. The control circuitry 670 can be part of thememory device 600 as shown or separate from the memory device 600.

The flash memory device illustrated in FIG. 6 has been simplified tofacilitate a basic understanding of the features of the memory and isfor purposes of illustration only. A more detailed understanding ofinternal circuitry and functions of flash memories are known to thoseskilled in the art. While the block diagram of FIG. 6 shows the controlcircuitry as being part of the memory device integrated circuit 600,alternate embodiments might have a memory array that is separate fromthe control circuit.

In one embodiment, all data manipulation, programming, and reading isperformed in the digital domain without converting the digital data bitpatterns to their equivalent voltage levels. An alternate embodimentperforms these functions as voltages that are converted withanalog-to-digital converters prior to manipulation by a separatecontroller. The controller then generates a digital signal correspondingto a bit pattern that is converted to a voltage level of an analogsignal by a digital-to-analog converter for programming into theaddressed memory cell or cells.

CONCLUSION

In summary, one or more embodiments of the present disclosure providecontinual adjustment of the memory device programming window as themaximum reliable erase threshold voltage and maximum reliable programthreshold voltage change due to erase/program cycling or othermechanisms. The present embodiments generate subsequent programmingwindows in response to program window changes.

The present embodiments optimize the program window over many blocks ina memory device. For example, one memory block may have a program windowfrom −2V to +3V while another program block in the same memory devicehas a program window from −3.5V to +4V. The present embodiments make theinitial program windows more uniform across the memory device due todifferences in the memory blocks.

Not only can the embodiments of the present disclosure change themargins between levels in an adjusted program window but the quantity oflevels can be adjusted as well. As the program window expands orcontracts, the quantity of levels within that window can also beexpanded or reduced as desired.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement that is calculated to achieve the same purpose maybe substituted for the specific embodiments shown. Many adaptations ofthe invention will be apparent to those of ordinary skill in the art.Accordingly, this application is intended to cover any adaptations orvariations of the invention. It is manifestly intended that thisinvention be limited only by the following claims and equivalentsthereof.

1. A system comprising: a processor configured to control the system;and a memory device coupled to the processor, the memory devicecomprising: an array of memory cells; and control circuitry configuredto control the memory device, wherein the control circuitry isconfigured to determine an initial programming window, prior to an eraseoperation, that encompasses all programming levels of a group of memorycells, the control circuitry further configured to determine first andsecond threshold voltages, and the control circuitry further configuredto adjust the initial programming window in response to the first andsecond threshold voltages such that the programming levels are moved inthe programming window as the programming window is adjusted.
 2. Thesystem of claim 1 wherein the array of memory cells is an array ofnon-volatile memory cells in a NAND architecture.
 3. The system of claim1 wherein the erase operation is a block erase.
 4. The system of claim 1wherein the first threshold voltage is a most positive threshold voltageof the group of memory cells.
 5. The system of claim 1 wherein thesecond threshold voltage is a most negative threshold voltage of thegroup of memory cells.
 6. The system of claim 1 wherein the group ofmemory cells is a memory block.
 7. A memory device comprising: an arrayof memory cells; and control circuitry configured to control the memorydevice, the control circuitry configured to read erase threshold voltageof memory cells in a group of memory cells, identify a maximum erasethreshold voltage for each memory cell in the group, generate a programlevel conversion table in response to the maximum erase thresholdvoltages, the control circuitry further configured to program referencememory cells in response to the conversion table.
 8. The memory deviceof claim 7 wherein the control circuitry is further configured tocontrol an erase operation of the group of memory cells.
 9. The memorydevice of claim 7 wherein the control circuitry is configured to programonly one reference memory cell per program level.
 10. The memory deviceof claim 7 wherein the control circuitry is configured to programmultiple reference memory cells per program level.
 11. The memory deviceof claim 7 wherein the control circuitry is configured to program eachreference memory cell with a reference fixed bit pattern that isassociated with an updated converted data bit pattern that isrepresentative of each program level.
 12. The memory device of claim 7wherein the control circuitry is configured to read the group of memorycells and read the conversion table with every read of the group ofmemory cells.
 13. The memory device of claim 12 wherein the array ofmemory cells is configured to output a signal corresponding to amultiple bit reference fixed bit pattern for each memory cell that isread.
 14. A method for adjusting a programming window in a memorydevice, the method comprising: determining an initial erase thresholdvoltage for a memory cell prior to an erase operation; adjusting theinitial erase threshold voltage to a subsequent erase threshold voltagein response to at least one erase operation; generating a program windowin response to the adjusted initial erase threshold voltage and anadjusted initial program voltage; and determining a number of programlevels that fit within the program window.
 15. The method of claim 14wherein the program window is defined by a most negative thresholdvoltage and a most positive threshold voltage.
 16. The method of claim15 wherein the most negative and most positive threshold voltages aredigital bit patterns.
 17. The method of claim 14 wherein each programlevel is represented by a multiple bit digital bit pattern.
 18. Themethod of claim 14 and further comprising adjusting the initial erasethreshold voltage subsequent to each of a plurality of erase operations.19. The method of claim 14 wherein adjusting the initial erase thresholdvoltage to a subsequent erase threshold voltage comprises increasing theinitial erase threshold voltage to a higher subsequent erase thresholdvoltage.
 20. The method of claim 14 wherein each program level isrepresented by a voltage.